Our aims are to conclusively demonstrate that energy expenditure due to physical activity is critical in the prevention of weight gain and to measure the amount of physical activity needed to maintain weight. By taking the difference between total energy expense from doubly-labeled water and resting metabolic rate plus thermic effect of a meal, we can obtain energy expended in physical activity in free-living subjects. Thus we can, for the first time, quantitate aspects of lifestyle in the subject's home environment.